Traditionally, in separating air, if argon is to be obtained as a product gas, the incoming air is separated into relatively pure streams of oxygen, nitrogen and argon. European Patent Application No. 136 926A, for example, discloses the operation of a conventional double column with argon "side-draw" for producing nitrogen, oxygen and argon products. In the process disclosed in the European Patent Application, advantage is taken of a temporary fall in the oxygen demand in order to increase the production of one or more of the other products, for example argon. A liquid is thus taken from one of the two columns forming the double column and is passed to the top of an auxiliary column or mixing column operating at substantially the pressure of the low pressure column. A gas whose oxygen content is less than that of the liquid is taken from the low pressure column and is passed to the bottom of the auxiliary column. A liquid collected at the bottom of the auxiliary column is passed as reflux into the low pressure column at substantially the level from which the said gas is taken. As more oxygen-rich liquid is taken from the double column and passed to the auxiliary column, more reflux may be provided for the low pressure column, thereby making possible an increase in the rate of argon production. However, this method involves substantial inefficiencies which makes it unsuitable for use in a plant for producing argon as the primary or sole product of air separation.
In our copending British patent application No. 8611536 there is disclosed to a method of separating argon from air in which an improvement in the operation of the auxiliary or mixing zone is made possible. In the mixing zone, a liquid flow and an opposed vapor flow are established which become progressively richer in nitrogen and oxygen, respectively. A mixed waste stream containing both nitrogen and oxygen is withdrawn from an intermediate point of the mixing zone and fluid therefrom is utilized to provide heat transfer in the process. The present invention relates to a process and apparatus for separating argon from air which enables further improvement to be obtained in the operation of the mixing zone.